"In Science the authority embodied in the opinion of thousands is not worth a spark of reason in one man." - Galileo Galilei

Thursday, June 12, 2008

A Swift Kick in the ASP

Gary Taubes' Good Calories, Bad Calories provided a nice and readable description of the current understanding of fat metabolism, in particular the major mechanism of how dietary calories wind up in fat cells, and how stored fat is made available for energy. The mechanism is fairly simple, and is a scientific "fact" as much as there ever can be one (lots of supporting evidence, no alternative hypotheses). Dietary fats, as well as those created in the liver from carbohydrates, are transported around the body in large molecules called lipoproteins. We've all been inundated with propaganda about lipoproteins, e.g. low-density lipoprotein (LDL) is "bad cholesterol", high-density lipoprotein is "good cholesterol", very low-density lipoprotein (VLDL) is "triglycerides", which are also "bad". The popular nomenclature is terrible and confusing.

Lipids are substances like fat and cholesterol which are not water soluble. To be carried in the blood (which is mostly water), lipids are carried inside of large lipoprotein molecules, which basically wrap up a droplet of lipids in a protein coat. Protein is water soluble, problem solved. The specific proteins on the surface of the lipoprotein allow it to bind to various receptors, so different lipoproteins can perform different functions, depending on receptor binding. Thus, cellular LDL receptors grab LDL from the blood so the cells can extract cholesterol, while HDL bind to receptors that allow it to take away "used" cholesterol for recycling in the liver, e.g. when cells die.

Most of the fat transported by lipoproteins is in the form of triglycerides (more technically known as triacylglycerol), a largish molecule consisting of three fatty acids attached to a "backbone" molecule of glycerol. Two kinds of lipoproteins carry most of the triglycerides: chylomicrons and VLDL. Chylomicrons are manufactured in the intestinal lining, packaging up digested fatty acids and cholesterol. The chylomicrons are (for reasons unknown to me) transported through the lymphatic system and dumped into the blood via the thoracic duct. Cells then have the opportunity to grab fat or cholesterol from the chylomicron, and some other changes happen to the surface proteins which rather quickly render it a chylomicron remnant. The liver vacuums up chylomicron remnants and repackages any lipids as VLDL (which also carries fat created by the liver from excess glucose). The VLDL then returns to the blood, and again cells can grab fats as necessary.

The triglyceride molecules carried by chylomicrons and VLDL are too large to pass across the cell membrane. In order to get some fat into a cell, the individual fatty acids must be released from the tryglyceride; fatty acid molecules can cross the cell membrane. The primary enzyme which performs this tasks is lipoprotein lipase, or LPL.

So that (long-winded) explanation gets us through part one of how fat is stored: LPL frees fatty acids from triglycerides in lipoproteins so they can get inside of the fat cells. Now, fat cells don't store fatty acids directly, but instead create their own triglycerides. However, the glycerol molecule itself also cannot cross the cell membrane. Instead, the fat cells ultimately make their own glycerol (actually a substance known as alpha glycerol phosphate) from glucose, which in turn must be supplied by the blood. Fat storage thus requires two crucial ingredients: action of LPL on chylomicrons or VLDL to free fatty acids, and availability of glucose including the ability to transport that glucose from the blood into the fat cell, which requires some specialized molecules called glucose transporters, or GLUTs.

Now Taubes points out that the primary control mechanism for both LPL activity and glucose transport is the hormone insulin. More insulin means more LPL and more glucose transport, thus more fat storage. Additionally, inside the fat cell lives an enzyme called hormone sensitive lipase, or HSL. HSL performs the same essential task as LPL, but from inside the fat cell: it frees fatty acids from stored triglycerides, so they can be made available to the blood (being carried away bound to the blood protein albumin). HSL response to insulin is opposite of LPL: less insulin means more HSL activity. So when insulin is high, fat tends to be stored, and when it is low, fat tends to be released. It's a nice tidy story, and gives a biochemical basis for the hypothesis that overconsumption of carbohydrates is what drives most obesity. Eating carbs not only raises insulin, it also makes available lots of glucose, thus supplying both of the critical ingredients for fat storage, while simultaneously suppressing the release of fat from fat cells.

I like this story, but have long had the nagging suspicion it is not complete. Consider, for example, the Inuit, whose traditional diet consists almost entirely of protein and fat. Protein does raise insulin. Insulin is the sort of the "key" for opening cells the macronutrients (protein, fats, and carbohydrates). Even if you don't eat any carbs, you need insulin to go up in response to protein consumption so your cells can take up the constituent amino acids and use them for building tissue, making functional proteins like hormones, etc. Protein consumption also triggers the pancreas to secrete another hormone called glucagon, which amongst other things blocks the entry of glucose into cells.

So, naively, a meal containing only fat and protein is somewhat blocked from having the fat stored, because glucagon inhibits the fat cells from taking in the glucose required to build triglycerides. But you do need to store some fat. Fat cells are a sort of energy reservoir, providing a steady source of energy between meals, so even if you eat zero carbohydrates, there should be a mechanism for storing a bit of fat. My guess was that this was accomplished through a precise balance of insulin, glucagon, and blood glucose. And it has to be precise, because too little storage and you run out of gas, but too much and you get fat and slow, making it more likely that you become polar bear food. But biological systems are rarely precise, rather achieving balance through robustness rather than precision. It also seemed like there should be some dose dependent mechanism for fat storage, e.g. eat more fat, store more fat. We certainly evolved that mechanism for storing away energy from carbohydrate-rich meals, and it seemed that something similar should be in place to take advantage of fat-rich meals, like bone marrow.

So this post at the Emotions for Engineers blog caught my attention, because at one point it mentions an alternative metabolic pathway fat storage. Sounded juicy, so I dropped a comment asking for elaboration, and was directed to information on acylation stimulation protein, or ASP. There seems to be a fair amount of confusion both in the scientific literature and on the Internet as to exactly how/why ASP did it's thing, and the implications for obesity. I did a big of digging, and though I certainly haven't solved the mystery, I did uncover some clues. This paper, in particular, provides a lot of useful information.

Fat tissue is increasingly recognized as an endocrine organ, generating several hormones related metabolism. You've probably heard of leptin. When fat cells expand from storing fat, they release leptin. Leptin does several things, most notably sensitizing other parts of the body such as the hypothalamus to the effects of hormones affecting satiety and gastrointestinal activity (see this excellent review for more). In short, when fat cells store more fat, they release more leptin, which makes you less hungry, until the fat cells shrink causing them to release less leptin, allowing you to get hungry again. There are many different such mechanisms regulating energy storage, metabolism, and hunger, forming a robustly controlled system, one that works well across a wide variety of input conditions.

ASP is another hormone secreted by fat cells, with several effects. First, ASP can increase LPL activity, making fatty acids available for transport into the fat cells. Second, ASP increases the expression of glucose transporters in fat cells, allowing them to bring in the glucose required to store fat. So ASP plays roughly the same role as insulin in fat storage, but rather than being generated by the pancreas in response to carbohydrates, is generated by the fat cells themselves. Better yet, ASP stimulates the production of triglycerides inside the fat cells. But what causes ASP to be secreted?

The answer, at least in test-tubes, is chylomicrons. When fat cells are exposed to chylomicrons they generate lots of ASP. By contrast, exposing the same cells to glucose, fatty acids, VLDL, HDL, or LDL elicits little ASP response. Further, the ASP response exhibits both a time and concentration dependence on chylomicron concentration.

This is an important clue. As discussed above, chylomicrons are the first step in transporting dietary fats into the body. When you eat a lot of fat, you make more chylomicrons, which causes the fat cells to make more ASP, which stimulates greater fat storage. But the chylomicrons only hang around for a relatively short time, being converted in the liver to VLDL. The receptor for VLDL (VLDL-R), when activated, does increase LPL activity, but to my knowledge does not stimulate glucose transport into fat cells. Thus the fat in VLDL is available to be used for energy, because the LPL frees the fatty acids for transport across cell membranes; but without some other hormonal signal (e.g. insulin), rather little of this fat can be stored in adipose tissue.

Two questions then arise in the context of a low-carbohydrate/high-fat diet. The most obvious one is "can I get fat by eating too much fat?" Taubes lays out the case that overconsumption of carbohydrates drives fat storage through the action of insulin, but can overconsumption of fat do the same via the action of ASP? When viewed with the most narrow lens, the answer is clearly "yes". While insulin's effects on LPL and glucose transport are considerably stronger than ASP, ASP does ultimately trigger the same conditions leading to fat storage. So if you eat enough fat for a long enough time, in principle you will become obese.

But if we take a step back, things are not so simple. The body has many feedback mechanisms for regulating energy content, such as leptin secretion by large fat cells, leading to suppression of appetite. These mechanisms regulate feelings of hunger, metabolic rate, how fast the stomach empties, etc. The system has presumably evolved to be robust over a wide range of environmental and nutritional conditions, allowing us to have enough energy to make it through times between meals while not having to carry so much that physical performance and other health aspects are compromised. The whole chain of events described above provides a nice example. Eat lots of fat, intestines create lots of chylomicrons. Chylomicrons stimulate fat cells to make ASP, which in turn increases fat storage. As fat cells store fat, they release leptin, which suppresses appetite and sensitizes the body to other satiety signals. But chylomicrons are fairly quickly turned into VLDL, which do not stimulate fat storage, but do make fat available for energy. The brain can detect VLDL levels, and regulate gastric emptying, appetite, etc. until the fat in the VLDL is used up. And that's just one of a complex web of interactions between hormones, the nervous system, metabolism, and digestion.

To become obese (at least without trying really hard), some key regulatory mechanism needs to be broken. For instance, there is a genetic defect which causes the fat cells to not produce leptin. People (or mice) with this defect have an unstoppable appetite, and become extremely obese. Treating them with leptin can reverse this condition. Another example is Cushing's disease, which is a small tumor on the pituitary. The net effect of Cushing's disease is that it causes the body to have high levels of the hormone cortisol. I had a friend with Cushing's disease. He ran five miles every day, and by any measure ate a healthy diet, yet continued to gain weight. Why? Increased cortisol (from the sympathetic endocrine system) can cause compensatory secretion of insulin (from the opposing parasympathetic endocrine system). Chronically high insulin will make you fat no matter how much you exercise or how little you eat. Keep insulin high, and you can literally starve to death while remaining obese.

But it appears the big hitter is carbohydrate consumption, particularly refined carbohydrates. These cause both drastic increases in insulin levels and make available lots of glucose for triglyceride storage. Though insulin nominally acts to suppress appetite and GI motility, high levels drive energy nutrients out of the blood and into the cells, ultimately leading the brain to "override" other mechanisms such as leptin, because low levels of energy nutrients in the blood basically signal imminent starvation; indeed, the brain itself needs a certain level of blood sugar to be maintained for proper operation. So eating carbs not only causes you to efficiently store fat, it also drives you to eat more food, and that food is typically more carbs to stabilize your blood sugar, leading to a vicious cycle.

I don't see a similar issue when eating a high-fat/low-carb diet. Fat ingestion does not cause hormonal derangement. Energy levels in the blood are maintained, allowing the various appetite regulation mechanisms to operate normally without getting an emergency override to eat more food despite available energy in the body. ASP production is stimulated only by chylomicrons, which are relatively short-lived, allowing a limited amount of dietary fat to be stored, while the rest is made available as energy. In principle, you could get fat by eating enough fat, but in practice it would probably be very difficult. You would have to force yourself to eat even though you felt extremely full, and continue to do so over a long time period. Not impossible, but definitely an uphill battle against a whole host of hormonal and nervous control systems, very much the analog of trying to lose weight on a low-fat/high-carbohydrate diet.

While it may be hard to gain fat through a high-fat diet, it is likely possible to keep on a certain level of body-fat. Low-carbohydrate diets are known to "stall", where the last 20 or so pounds just won't come off, regardless of carbohydrate restriction. I suspect our friend ASP plays a crucial role here. The low insulin levels on a low-carb diet will allow the fat cells to free fatty acids, but if you are consuming enough fat, at some point this effect will be balanced by that of ASP, and voila, no more fat loss.

20 comments:

Anonymous
said...

What a great article! But I have a question for you: if ASP triggers fat storage, but fat storage requires a glycerol molecule (in order to bind the fatty acids into triglycerides) -- and you're not consuming any glucose to make the glycerol from -- how does ASP occur?

It sounds like the glycerol has to come from glucose produced through gluconeogenesis, which in turn burns fatty acids. If that's the case, doesn't ASP still result in a net fat loss?

Hi Michael. I would agree that the required glucose must come from gluconeogenesis. Since gluconeogenesis is fueled by fatty acids, some of the fat you eat must be used for energy for this purpose, and of course can't be stored. Off the top of my head, I don't know how many fat calories are required for this. I'll bet Michael Eades blog has the answer somewhere.

Anyway, I believe biological systems evolve to maintain reasonable balance over a broad range of conditions. When everything works correctly, several control mechanisms can work in concert to maintain the balance point. If all is working properly, average bodyfat stores should be roughly constant over long time periods. My guess is that the ASP mechanism at least partly helps maintain bodyfat stores at a reasonable level, because it looks to me like it "plays nice" with many other regulatory processes. By contrast, excess carbohydrate consumption tends to usurp these control mechanisms, so it should be no surprise that the body's response is to move away from a healthy metabolic balance (e.g. stores excess fat).

It's all just theorizing. Hopefully future research will nail this down with greater precisision.

I'm going to try and stay away from giving specific recommendations on this blog, instead focusing on providing information to help individuals make their own health and nutrition decisions. So I'll lay out some thoughts on what might cause the low-carb stall, which should give you some ideas of things to try.

We know insulin is the major player in determining whether fat in adipose tissue remains stored or is freed for use. Higher insulin "locks in" stored fat. We know that carbs raise insulin, but insulin levels may be affected by other things as well. There's a fair amount of evidence for the "cephalic insulin response", which is insulin release under direct control of the brain, particularly when tasting something sweet. It might be that artificially sweetened foods cause enough insulin release to stall fat loss. I can relate my own experience here, where I stalled for over a year. I cut back a lot on diet soda because of concerns about aspartame, and dropped another 15 pounds.

Stress can raise insulin levels. Stress hormones come from the sympathetic endocrine system, while insulin is the primary hormone of the parasympathetic endocrine system. The body tends to adjust hormones from the two sides to maintain some sort of balance, so higher levels of stress hormones can lead to chronically increased insulin. See Robert Sapolsky's excellent book "Why Zebras Don't Get Ulcers" for more info.

Other substances found in foods can bind to insulin receptors. One class of these is amines, and in particular it has been shown that tyramine stimulates glucose uptake and probably other insulin-like effects. People taking a class of drugs called monoamine oxidase inhibitors (MAOI) are supposed to avoid foods high in tyramine, so we can look to lists of "bad" foods for MAOI patients (see e.g. here and here) to get some hints of foods that might stimulate insulin-like effects. Some foods contain a class of proteins called lectins, and some lectins also have insulin-like activity. From what I've been able to gather, these are often found in seeds, including grains and probably nuts, though I haven't found much evidence that nuts specifically have significant insulin-like effects.

The ASP mechanism would seem to indicate that at some point a high-fat diet is going to hit an equilibrium point, where on average stored fat remains constant. More study is needed on the details, but it makes sense, and lends some credence to the idea of a "fat fast". Obviously, even if you cut fat for awhile carbs still need to stay low to keep insulin in check, and a low-carb/low-fat diet is almost certainly calorie-restricted as well. Over time, eating lean-protein with no other calorie source is unhealthy (Google "rabbit starvation"), so should you choose to do this, keep it restricted to short duration and you should probably consult a doctor.

Typical low-carb snacks like nuts and cheese not only contain possible insulin-like compounds, they're very high fat. So these may represent a double-whammy.

It may be possible to affect your insulin sensitivity through exercise, particularly resistance training.

BTW, I'm the co-author (along with Dr. Eades) of the Slow Burn Fitness Revolution. I can attest to the power of resistance training for controlling blood sugar. So may of my clients have benefited greatly by including it into their low carb regimen.

Hi Fred. Thank you for the praise. I've read your book, and am a BIG fan of slow burn. Just need to do it more than once a month :-) It's a kind of a catch-22 when it takes more time to go through the pre/post-workout stuff than the workout itself. Maybe I need to try the home version :-)

Good, intelligent work here. Barry Sears, in The Age-Free Zone makes a claim that the body, when insulin levels fall too low, becomes more efficient at storing body fat... "In addition, without adequate supplies of glucose, you will become irritable and mental cognition decreases. In addition, research has indicated that the longer you stay in ketosis, the more your fat cells adapt so that they are transformed into ‘fat magnets,’ becoming 10 times more active in accumulating fat."

Unfortunately, he doesn't elaborate on this despite elaborating on practically everthing else. Experientially, I cannot argue against this, as a friend of mine who has always been underweight, switched from an extremely high-carb diet to a low-carb diet for the emotional benefits, and threw on about 30 pounds in less than a year, most of it fat. Also, any diet with too little carbohydrate slows down the metabolism, just like creating a calorie deficit doing the eat less exercise more thing. Even ole' Bob Atkins knew that, which he mentions on page 303 of Dr. Atkins New Diet Revolution...

“…remember that prolonged dieting [including ‘this one’] tends to shut down thyroid function. This is usually not a problem with the thyroid gland but with the liver, which fails to convert T4 into the more active thyroid principle, T3.

That's certainly a key indicator as to why the last 20 pounds can't be lost. A body with a really low metabolic rate induced by calorie deficit and/or carbohydrate starvation always resists fat loss, it just takes a while for the metabolism to slow down enough to halt the process. Ketosis also raises cortisol.

Why didn't this happen to Eskimos? They weren't in ketosis. They were efficiently converting protein to glucose, like a carnivore. Ketosis causes enhanced fat storage and reduced thyroid activity.

I'd be suspicious of claims like "10 times more active in accumulating fat" that are not backed up by some biochemistry. But I would think fat cells do become more efficient at storing fat as average insulin levels drop, restoring insulin sensitivity. Fat cells would also become more efficient at releasing fat as well, and so I would expect the body to be better overall at maintaining energy balance.

Was their any biochemical basis given for the idea of thyroid dysfunction on "any diet"? That strikes me as curious. I'd also wonder what the baseline measurement of thyroid function is based on. If it's from the general population eating the standard American diet, it may not represent a healthy state. This type of thing may be a frequent problem. I had an extended discussion on low-carb diets with an MD friend, who was very worried about the changes in blood pH. But again, the baseline blood pH is taken from a population eating a high-carb diet, so may not represent "normal".

As for low-carb stalling on the last (10 or) 20lbs - it can. A lot of people become enamored with the fact that they can acheive fast fat loss with no attention to portion control and get in the habit of eating way too much. Then when they hit the home stretch they stop losing.

At that point if you introduce them to the concept of listening to their actual appetite they can lose the last few pounds. The nice thing with the low carb is that the people who tend to have trouble with being obese get addicted to carbs and eating carbs gives them a voracious appetite that they literally can't control.

In my experience men can get down to about 10% body fat as a new base line without counting calories. 10% is nice, and VERY nice if you've been walking around at 25-30% before!

Why do not limit the protein to the optimal amount - 1 g per kg of ideal/desired weight to keep the gluconeogenesys in check, keep carbs low and fat still high to allow losing the last lbs? Kwasniewsky of the optimal diet fame advocates using this formula for health maintenance1g protein - 0.5 g carb - 2.5-3.5 g fat

and this for wight loss:1 g protein - 0.5 g car - 1.5 g fat

I am a 5'3'' female who wants to be 110 lb. I would eat 50 g protein which is 100% RDA by the way, 25 g carbs that is close to Atkins induction and 75 fat. Total calories would be 1000 - great for dieting with any nutritent mixturen, protein is sufficient, carbs are low and fat enough to not allow thyroid shut down.

Well ASP would nicely explain why some of us zero carbers tend to gain minor but noticeable weight seemingly due to exercise, especially distance running (weight drops when exercise stops). Anecdotally I have seen that even if you gorge on fat you won't necessarily gain weight, but add in running and the subsequent increased appetite and consumption to meet it, and on comes 5-10lbs. I suspect that the increased demand on fatty tissue to release fat somehow stimulates or increases ASP production. This seems like a good idea for the body to do since you don't want to run out of fat -- primitive/paleo hunting being an aerobic exercise that can take a lot of time, the hunter with more fat may have an easier time surviving the "lean times" despite an increased activity level.

The exercise-related weight gain you mention is an interesting phenomenon. Do you know if it's fat or not? I.e. does body-fat percentage increase? If not, one alternative might be that muscle tissue is storing more glycogen in response to exercise. Glycogen requires a lot of water for storage.

I'm studying metabolism as it relates to fasting. I have a question regarding your post above. You said,

"Protein consumption also triggers the pancreas to secrete another hormone called glucagon, which amongst other things blocks the entry of glucose into cells."

Could you point me to a reference for this? I thought that glucagon release is triggered only by low glucose levels.

I also thought that glucose uptake is competitively blocked by free fatty acids themselves, not glucagon per se. Is this how you meant it? i.e. that glucagon ups fatty acids release, high levels of which in turn block the glucose uptake. Or is there another mechanism through which glucagon does it? It's very important to me that I get it right.

I think you can find some discussion of this in metabolism textbooks as well, e.g. Frayn.

You may be right on the second point. I'm not able to find where I got the idea that glucagon directly blocks glucose uptake by adipocytes (I believe it actually stimulates glucose release in the liver). I'll let you know if I find the original source.

Based on the liver providing the mechanism for converting ASP to VLDL, it adds to the evidence that liver disfunction can lie at the heart of a fat loss stall. High fructose consumption can cause non-alcoholic fatty liver disease which could compromise liver function, allowing the ASP to stick around longer than it should. However you mention there are layers of feedback mechanisms so maybe this would be dealt with as well.

I also wanted to comment on the distance running causing fat retention - distance running often raises cortisol and you mentioned high cortisol resulting in high insulin.

I'm not sure how authoritative the following link is, but it explains sufficiently clearly for the lay person, with enough detail (probably) for the expert:

http://www.medbio.info/Horn/Time%203-4/homeostasis_2.htm

See especially the bit about low-carb diet and insulin & glucagon response to protein. Seems to me that there is at least some potential for fat storage (independently of what may be going on with ASP).

ISTR that in GC, BC, GT didn't seem to think ASP was all that important, but of course the science may have moved on.

You have to have some way of storing fat even on a low-carb diet, otherwise you'd die. Fat is medium and long-term energy storage, to get you through not only lean times, but also those times when you're not eating (like during sleep). The relevant question is how/if one can effect a breakdown in overall metabolic regulation, such that the various appetite controls fail and you wind up storing excess fat in the long term. It's certainly physically possible, i.e. I would expect if you force-fed somebody 10,000 calories/day of only fat/protein, they would gain excess fat.

... Ugh. I bought the 'why zebras don't get ulcers' on the strength of how much I enjoyed your blog. Second page of the preface contains pandering about how red meat is scary and talking about the influence of the mind on the body with what feels like implied mind body duality. I really hope this gets better :(